Project Summary Visual perception is mediated by complex interactions amongst neurons in the retina, visual cortex, and subcortical brain structures. The importance of vision to humans and other primates is reflected in the enormous percentage of cerebral cortex devoted to processing visual information. Thus, deficits in visual processing are particularly debilitating and arise from abnormalities not only in the eye, but also in cortical circuitry. For example, strabismus or amblyopia during childhood can have long-lasting effects on the cortical circuits that process visual information. There is also evidence that some forms of dyslexia result from central visual system abnormalities. The function of the nervous system is dependent on complex interactions between networks of neurons composed of multiple neuron types across multiple cortical areas. The proposed studies are aimed at revealing the organization and functional impact of local and long-distance feedback connections within and between mouse visual cortical areas. These studies are conducted in mice to take advantage of the range of molecular, genetic, and viral tools that can be used to elucidate brain circuits and link them to function using optogenetic and imaging methods. The 2 aims will reveal: the fine-scale organization of V1 microcircuits linking cortical layers 2/3 and 5, and the functional impact of feedback from layer 5 to layer 2/3; and the organizational principles and functional impact of cortico-cortical feedback.